A recombinant "library" of the DNA of the African green monkey (Cercopthicus aethiops) in a bacteriophage lambda vector was constructed. Using purified DNA fragments as probes, segments of monkey DNA homologous to the control region around the origin of replication of simian virus 40 (SV40) were isolated. Three distinctly different monkey segments homologous to the control region of SV40 were detected and characterized by subcloning and primary nucleotide sequence determination. Each segment is only a few hundred base pairs long and contains multiple and disconnected sequences homologous to the control region of SV40. The number and arrangement of the homologous sequences in each of the three segments differs and is distinct from the arrangement in the virus. The segments include homology to SV40 DNA regions known to be involved in the initiation of viral DNA replication and the start of early transcription. Accordingly, one of the segments has been tested for its ability to replace the SV40 control region for these functions. Preliminary results indicate that the monkey segment contains transcription start sites.